Method of supplying fuel to a dual head combustion chamber

ABSTRACT

A method of supplying fuel to a combustion chamber of a turbojet engine having a low power head with a plurality of low power fuel injectors and a high power head having a plurality of high power fuel injectors is disclosed in which fuel is supplied to the plurality of low power fuel injectors during low power operation of the engine, fuel is supplied to a first fuel circuit in the plurality of high power fuel injectors during high power operation of the turbojet engine and fuel is also supplied to a second fuel circuit in the plurality of high power fuel injectors during low power operation of the turbojet engine, the second fuel circuit being separate from the first fuel circuit. This method of supplying fuel to the dual head combustion chamber optimizes operation of the combustion chamber in all modes of engine operation.

BACKGROUND OF THE INVENTION

The present invention relates to a method of supplying fuel to acombustion chamber of a turbojet engine having both a low power headwith a plurality of low power fuel injectors and a high power headhaving a plurality of high power fuel injectors.

Aircraft turbojet engines are required to operate in various modes bothat very high power output and at relatively low power output. Turbojetengines utilized in military aircraft must also minimize infra-redemissions in order to prevent detection of the presence of the aircraftduring any mode of operation. Accordingly, fume emissions from carbonparticles and infra-red emitting fumes from nitrogen oxides must bereduced. The carbon particles and fumes are produced predominantlyduring high power operations.

Dual head combustion chambers are known to reduce polluting emissions ofthe turbojet engine especially during high power operations. The highpower, or take off, head is optimized for full power operation and feedsa sufficiently lean fuel/air mixture to the combustion chamber to reducefume production and the formation of large quantities of nitrogenoxides. In low power operation, only the low power head supplies a richfuel/air mixture to the primary zone of the combustion chamber to ensureflame stability, thereby preventing engine flame-out. The richness ofthe fuel/air mixture produces large quantities of fume emissions duringsuch low power operations.

Typically, the high power or take-off head and the low power head areradially displaced from each other about the axis of the turbojetengine. This causes a non-homogeneous radial temperature distribution inthe gases emanating from the combustion chamber and contacting theturbine blades of the engine. Such non-homogeneous radial temperaturedistribution diminishes the useful life of the turbine blades.

The high power or take-off head is typically supplied with fuel onlybeyond 25% of the nominal engine thrust which causes the richness of thefuel/air mixture in the primary zone of the combustion chamber to dropmarkedly when operation of the high power head is initiated due to therelatively leaner fuel/air mixture than that supplied by the low powerhead.

It is known to supply a single head, conventional combustion chamberwith dual injectors comprising two coaxial tubes feeding fuel toseparate zones of the combustion chamber through separate fuel circuitsand separate air circuits. Such a system is illustrated in BritishPatent No. A 2,214,630 which describes a dual injector having twoseparate fuel circuits, a main feed circuit associated with a firstaxial swirler to feed a central zone and second feed circuit associatedwith a second axial swirler to feed an annular zone through channels inthe blades of the first swirler. The central zone is operative at lowpower while the annular zone operates only at high power.

French Patent No. A 2,421,342 also describes a double zone injectorwherein the central zone operates only at high power. These documentsare silent on the concept of such double zone injectors outfitting atakeoff head of a dual head combustion chamber.

SUMMARY OF THE INVENTION

A method of supplying fuel to a combustion chamber of a turbojet enginehaving a low power head with a plurality of low power fuel injectors anda high power head having a plurality of high power fuel injectors isdisclosed in which fuel is supplied to the plurality of low power fuelinjectors during low power operation of the engine, fuel is supplied toa first fuel circuit in the plurality of high power fuel injectorsduring high power operation of the turbojet engine and fuel is alsosupplied to a second fuel circuit in the plurality of high power fuelinjectors during low power operation of the turbojet engine, the secondfuel circuit being separate from the first fuel circuit. This method ofsupplying fuel to the dual head combustion chamber optimizes operationof the combustion chamber in all modes of engine operation.

The low power head and the high power head are radially displaced fromeach other about the longitudinal axis of the turbojet engine. Themethod enriches the fuel/air mixture in the primary zone of thecombustion chamber during the lower power mode of operation to be atleast 80% of the stoichiometric ratio. The fuel flow supplied to thesecond fuel circuit is controlled to remain between 40-50% of the totalfuel flow (Wf) supplied by the second fuel circuit and the low powerhead. Fuel flow through the first circuit of the high power fuelinjector begins when the engine reaches approximately 20% of its nominalground thrust.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial, cross-sectional view of a conventional dual headcombustion chamber for a turbojet engine.

FIG. 2 is a graph illustrating the fuel/air mixture richness (R) as afunction of the nominal engine thrust (FOO) in a conventional dual headcombustion chamber.

FIG. 3 is a graph similar to FIG. 2, but illustrating the fuel/airmixture richness (R) vs. nominal thrust (FOO) in a combustion chambersupplied with fuel according to the present invention.

FIG. 4 is a partial, enlarged view of a high power fuel injectorutilized in the method according to the present invention.

FIG. 5 is a partial, cross-sectional view taken along line V--V in FIG.4.

FIG. 6 is a partial cross-sectional view of a dual head combustionchamber supplied with fuel according to the method of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, 10 denotes a conventional, dual head combustion chambersupplied with compressed air through a diffuser 11 located downstream ofthe engine air compressor (not shown). The combustion chamber 10 isgenerally annular in configuration about longitudinal axis 12 and isbounded by inner wall 13, outer wall 14 and end wall 15 which connectsthe upstream ends (towards the left as viewed in FIG. 1) of the innerand outer walls 13 and 14. The combustion chamber exhaust passage 16directs the exhaust gases toward the blades of the engine turbine (notshown).

The combustion chamber 10 is located between an inner casing 17 and anouter casing 18 which are interconnected with the diffuser 11 and,together with the combustion chamber walls 13 and 14, define annularpassages 19 and 20 to direct the flow of primary air P feeding thecombustion chamber 10 through orifices 21 and to direct the flow of aircooling the air chamber walls 13 and 14.

A plurality of high power, or take-off injectors 22 and a plurality oflow power injectors 23 are located in the end wall 15 and are arrangedin radially spaced apart annular arrays around the longitudinal axis 12.A partition plate 24 is mounted to the end wall 15 between the highpower fuel injectors 22 and the low power fuel injectors 23,respectively, and extends inwardly into the combustion chamber generallytowards the exhaust passage 16. Plate 24 divides the upstream end of thecombustion chamber 10 into a first primary zone fed by the high powerinjectors 22 called the high power head 25, and a second primarycombustion zone fed by the low power injectors 23 and designated the lowpower head 26. The fuel injectors 22 and 23 are supplied fuel viaseparate fuel circuits 27 and 28 and each is associated with sets ofradial swirler blades 29 and 30 which are supplied with air through thediffuser 11, the air passing through the swirlers serving to vaporizethe fuel supplied through the fuel injectors. In such known dual headcombustion chambers, the low power fuel injectors operate during lowpower operating modes, whereas the take-off fuel injectors 22 aresupplied fuel only when the engine operation exceeds 25% of the nominalground thrust, FOO.

The curve C1 illustrated in FIG. 2 represents the fuel/air mixturerichness R in the primary zone in the vicinity of the low power head 26as a function of the nominal thrust FOO. The curve C2 illustrates thefuel/air mixture richness R in the primary zone in the vicinity of thehigh power head 25, while curve C3 illustrates the minimum fuel/airmixture richness corresponding to the lower operational limit of theengine. It is clear that the curve C1 drops sharply at approximately 25%of the nominal thrust FOO, at which point the high power head is beingsupplied with fuel. Below 25% of the nominal thrust, the fuel/airmixture richness in this primary zone exceeds the stoichiometric ratioof fuel to air (designated as 1 on the R scale) in order to ensure goodflame stability. However, such a level of fuel/air mixture richnessgenerates fumes which are detectable by infrared detectors. Beyond 25%of the nominal thrust FOO, the fuel/air mixture richness in the primaryzone is higher than 0.7, but is less than 1 on the R scale.

In the present invention, the high power head 25 of the above-describeddual head combustion chamber has a plurality of high power injectors 40with two separate fuel circuits. As best seen in FIG. 4, each high powerinjector 40 comprises a fuel injector portion 41 within which arepresent both a first fuel circuit 42 which supplies fuel to the highpower fuel injector during high power modes of operation and a separate,second fuel circuit 43 which supplies fuel to the high power fuelinjectors during the low power mode of operation.

An axial swirler 44 is mounted around the downstream end portion of thehigh power fuel injector 41. The axial swirler 44 is located inside acollar 45 which extends in a generally downstream direction (towards theright as illustrated in FIG. 4) from radial flange 46 and has adownstream frustoconical wall 47 flaring outwardly. A radial swirler 48is located downstream of the radial flange 46 as part of a bowl 49affixed to the combustion chamber end wall 15.

The second fuel circuit 43 passes through each of the vanes 50 of theaxial swirler 44 and communicates with orifices 52 formed in the collar45 such that fuel issues into the annular space 51 bounded by the collar45 and the bowl 49.

The collar 45 and the frusto-conical wall 47 divide the take-off head 25into two zones. The first zone is supplied fuel from the first circuit42 along with air passing through the axial swirler 44. The second zoneis supplied fuel from the second fuel circuit 43 and air through theradial swirler 48. In known fashion, the swirlers 44 and 48 may includecontrols so as to regulate the amount of air passing through each one.The zone receiving the larger air flow may be either the first or thesecond zone.

The low power fuel injectors 23 and the second fuel circuit 43 arejointly supplied with fuel beginning with the low power operating modesuch that the richness of the fuel/air mixture in the primary zone willbe at least 80% of the stoichiometric ratio, which will both ensureflame stability and avoid producing detectable fumes. The fueldistribution during the low power operating mode between the second fuelcircuit and the low power fuel injectors 23 is such that the second fuelcircuit 43 receives between 40% and 50% of the total fuel flow W_(f)supplied by the second fuel circuit and the lower power head. Beginningat approximately 20% of the nominal thrust and continuing through fullpower operation, both fuel circuits 42 and 43 of the high power fuelinjectors 40 will be simultaneously supplied with fuel.

FIG. 3 shows representative curves C'1 and C'2 of the fuel/air mixturerichness (R) of the primary zones of a dual head combustion chambersupplied with fuel in the above-described method. It will be noted thatthe fuel/air mixture richness of the low power head and of the highpower head are both less than the low power stoichiometric ratio. Such afuel/air mixture richness is approximately 1 when operating near 20% ofthe nominal thrust FOO and then drops to 0.7-0.8 near 30% of the nominalthrust, thereupon increasing through full power operation.

Because the two fuel injector heads are supplied during lower poweroperating modes, the invention increases the homogeneity of the fuel/airmixture as well as increasing the radial homogeneity of the exhaust gastemperatures to thereby increase turbine service life.

The foregoing description is provided for illustrative purposes only andshould not be construed as in any way limiting this invention, the scopeof which is defined solely by the appended claims.

We claim:
 1. A method of supplying fuel to a combustion chamber of aturbojet engine having a low power head having a plurality of low powerfuel injectors, a high power head having a plurality of high power fuelinjectors and a primary combustion zone, comprising the steps of:a)supplying fuel to the plurality of low power fuel injectors during lowpower operation of the turbojet engine; b) providing first and secondseparate fuel circuits in each of the high power fuel injectors; c)supplying fuel to the first fuel circuit in the plurality of high powerfuel injectors during high power operation of the turbojet engine; andd) supplying fuel to the second fuel circuit in the plurality of highpower fuel injectors during low power operation and high power operationof the turbojet engine.
 2. The method of claim 1 wherein fuel issupplied to the low power fuel injectors and the second fuel circuit ofthe high power fuel injectors during low power operation such that thefuel/air mixture in the primary combustion zone is at least 80% of thestoichiometric ratio.
 3. The method of claim 1 wherein the fuel flowsupplied to the second fuel circuit of the high power fuel injectors isbetween 40% and 50% of the total fuel flow supplied to the low powerfuel injectors and the second fuel circuit of the high power fuelinjectors.
 4. The method of claim 1 wherein fuel supply to the firstcircuit of the high power injectors commences at approximately 20% ofthe nominal ground thrust of the turbojet engine.
 5. The method of claim1, comprising the additional steps of providing first and second fueloutlets in the first and second circuits, respectively oriented suchthat fuel emanates from the second fuel outlet generally perpendicularlywith respect to the fuel emanating from the associated first fueloutlet.